ALLYL COMPLEXES OF PENTAAMMINEOSMIUM

Treatment of [Os(NH3)5(C3H5)2O](OTf)2 in methanol with triflic acid produces [Os(NH3)5(eta-3-C3H5)](OTf)3 in good yield. The cation in solution was characterized by H-1 NMR spectroscopy. In cyclic voltammetry, no redox activity is revealed within the solvent window, +1.5 to -1.5 V vs NHE. The complex readily undergoes addition at a terminal carbon by a variety of neutral and negatively charged nucleophiles. Complexes of cyclic allyls are generated when either the complex of anisole or of 1,4-dimethoxybenzene is hydrogenated in acidic methanol. As is true of the benzene analogue, hydrogenation does not proceed beyond two molecules of H-2/molecule of ligand. In the reduction of the complex of 1,4-dimethoxybenzene, elimination of methanol leads to the eta-2-benzene complex as the reaction product. A proton is eliminated from [Os(NH3)5(eta-3-C6H9)]3+, yielding [Os(NH3)5(eta-2-1,3-cyclohexadiene)]2+, and the reaction is governed by an equilibrium quotient of 0.3 M in methanol. This equilibrium quotient, together with that for the addition of CH3O- to coordinated allyl, leads to an estimate of 2 x 10(15) for the equilibrium quotient for elimination of CH3OH from [Os(NH3)5(eta-2-3-methoxycyclohexene)]2+, to be compared to ca. 0.1 for the free organic ligand. The comparison shows that the affinity of [Os(NH3)5]2+ for the carbon/carbon double bond in a conjugated diene is very much greater than for the carbon/carbon double bond of 3-methoxycyclohexene.